The Rosary has always been a weapon against heresy and trouble; in fact, the 7 October 1571 victory of Christendom over Islamic warriors at the Battle of Lepanto -- the first naval victory against the infidels -- was attributed directly to the Rosaries prayed by the faithful that they trusted in Virgin Mary and her powefull intersection.
A partial indulgence is gained, under the usual conditions, when praying a third of the Rosary (5 decades) continuously (i.e., one can't say a decade, go wash the dishes, and return to say the other decades).
A plenary indulgence is gained, under the usual conditions: the christian must be in a state of grace, have heard Mass and have taken communion, when it is prayed by a family group or publicly in a church or oratory. The decades' Mysteries must be announced, and the Mysteries meditated on.
